Hepatobiliary Surgery - part 2 ppsx

A Typical ductal anatomy,B triple confluence, C ectopic drainage of a right sectoral duct into the commonhepatic duct C1, right anterior duct draining into the common hepatic duct; C2,ri

Fig 1.12 Normal and aberrant sectoral ductal anatomy (A) Typical ductal anatomy,(B) triple confluence, (C) ectopic drainage of a right sectoral duct into the commonhepatic duct (C1), right anterior duct draining into the common hepatic duct; (C2),right posterior duct draining into the common hepatic duct, (D) ectopic drainage

of a right sectoral duct into the left hepatic ductal system (D1, right posterior sectoralduct draining into the left hepatic ductal system; D2, right anterior sectoral ductdraining into the left hepatic ductal system, (E) absence of the hepatic ductconfluence, (F) absence of right hepatic duct and ectopic drainage of the rightposterior duct into the cystic duct Reprinted with permission from: Surgery of theLiver and Biliary Tract (3rd Edition), Blumgart LH, Fong Y (Eds.) W.B Saunders,London, UK (2000)

19Essential Hepatic and Biliary Anatomy for the Surgeon

1

Fig 1.13 Normal and aberrant segmental ductal anatomy (A), variations of Segment

V, (B) variations of Segment VI, (C) variations of Segment VIII, (D) variations of Segment

IV Note there is no variation of drainage of Segments II, III, and VII Reprinted withpermission from: Surgery of the Liver and Biliary Tract (3rd Edition), Blumgart LH,Fong Y (Eds.) W.B Saunders, London, UK (2000)

Hepatobiliary Surgery, edited by Ronald S Chamberlain and Leslie H Blumgart.

©2003 Landes Bioscience

Imaging of the Liver, Bile Ducts

and Pancreas

Douglas R DeCorato, Lawrence H Schwartz

Imaging of the upper abdomen including the liver, biliary tree and pancreas hasevolved rapidly over the past ten years Fast imaging techniques using both com-puted tomography (CT) and magnetic resonance imaging (MRI) permit dynamiccontrast-enhanced imaging in seconds Similarly, ultrasonography (US), an integralpart of hepatobiliary imaging for both benign and malignant disease, has seen markedadvances due to improved technology Modalities such as nuclear medicine andangiography play a more specialized role in the identification and characterization

of pathology For example, although nuclear medicine is limited in its evaluation ofhepatic lesions, it is a physiologic examination that can aid in detecting disease ofthe gallbladder, biliary tree, and postoperative complications such as bile leaks.These three imaging modalities, which are utilized most commonly, have certaincharacteristics that are unique to each A detailed discussion of the physical proper-ties and principles underlying each imaging technique is beyond the scope of thischapter, and interested readers are referred to the additional reading section at theend of the chapter Basic principles will be outlined below

US is based on the transmission or reflection of sound waves (echoes) as theypass through tissue Structures are described with regard to their relative echogenicitycompared with surrounding structures A hyperechoic lesion has more echoes thanits surroundings and thus appears slightly brighter than the adjacent tissue Ahypoechoic mass has fewer echoes and thus appears darker

CT is based on the absorption of x-ray by structures This absorption undergoes

a mathematical calculation to generate specific attenuation values in Hounsfieldunits (HU) Structures are referred to as high or low attenuation or hyper- orhypodense lesions A hyperdense or high attenuation lesion appears brighter thanbackground structures or has been enhanced; a low attenuation or hypodense lesion

is darker than background structures It cannot be assumed that a low attenuationlesion has not been enhanced, or a high attenuation lesion has been enhanced Spe-cific criteria exist for enhancement by CT based on an increase of 10 HU from anoncontrast study to a contrast study

MRI uses a strong magnetic field to image innate protons within organs Signalintensity is based on sequence parameters and the relative abundance of protons.Commonly used terms for MR sequences include T1, T2, and gradient echo Similar

to US and CT, structures are described as hypointense and hyperintense T1-weightedimages will display fluid as dark (hypointense), and are classically thought of as

21Imaging of the Liver, Bile Ducts and Pancreas

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displaying anatomic information Certain MR contrast agents, such as gadoliniumchelates, are administered with T1-weighted images (mostly gradient echo images, asequence that can be adjusted to generate rapid T1-weighted images) T2-weightedimages generate bright (hyperintense) fluid and are generally used to identify patho-logic processes

Imaging of the hepatobiliary system is dependent on the suspected underlyingdisease, pretest likelihood of a positive study, and clinical suspicion of concurrentunderlying disease process (for example, cirrhosis) The initial study, if chosen prop-erly, may be able to both identify the underlying disease process and provide staginginformation at the same time In a patient with a low pretest likelihood of disease,initial screening of the liver is best accomplished with US due to its wide availabilityand relatively low cost A patient with a negative ultrasound, and low pretest likeli-hood of disease, may need no further workup However, patients with a high pretestlikelihood of disease may require a different initial examination Once a lesion orunsuspected underlying parenchymal disease is identified on US, a second imagingtest may be necessary A comparison between the three main imaging modalities issummarized in Table 2.1

Test Selection

The choice of initial imaging test is influenced by several factors, including patientpopulation, prevalence of certain disease processes, test availability, cost restraints,radiologic equipment and the pretest likelihood of a positive study For example, US

is an excellent screening test for many patients; however, body habitus, underlyinghepatic disease, and overlying bowel gas can limit an otherwise diagnostic examina-tion CT with contrast routinely delivers images of diagnostic quality; however,precontrast images, as well as arterial dominant phase images, are not always obtained.Significant limitations arise in patients in whom intravenous contrast is contraindi-cated or not administered for other reasons A noncontrast CT scan markedly limitsidentification of small lesions and hinders characterization MRI routinely acquiresmultiple sequences that aid in lesion identification and characterization MRI has aset of absolute contraindications for scanning (Table 2.2) Additionally, since noradiation is administered, precontrast images are routinely obtained MRI scannersare variable in both speed and quality Low field strength systems and “open” mag-nets may not afford the resolution necessary for preoperative planning

Contrast administration is an important aspect of both CT and MRI CT scansprincipally use iodinated contrast material, while MRI has a variety of contrast agentsavailable of which gadolinium is the most common Patients allergic to iodine willusually not cross-react with gadolinium-based agents and, thus, may be safely referredfor MRI A variety of newer MRI contrast agents, including tissue specific antigens,are currently under investigation

A more detailed analysis of the best test for each specific hepatic, biliary andpancreatic abnormality is beyond the scope of this Chapter, but general guidelinesare discussed below and summarized in Table 2.3

Evaluation of the liver is best done with MRI, as it provides improved lesiondetection and characterization In addition, MRI can precisely define underlyingliver processes, such as fatty infiltration, which can sometimes be confused with otherdiagnoses on both CT and US An additional advantage of MRI is the development of

Table 2.1 Comparison of imaging modalities

Resolution Superior contrast Superior spatial Spatial resolution

transducer selected

Contrast material Superior safety High safety profile No contrast

profile than CT of nonionic administeredand lower volumes contrast Requires

are administered relatively high

doses of iodinatedcontrast for study

Radiation None Ionizing radiation None

used

Speed Rapid imaging Rapid imaging Imaging time

time available time Entire exam operator dependent

may be performed

in under a minute

Technique Variable depending More uniform than Standard images

on hardware and MR, while still with additionalsoftware somewhat variable images acquired

depending on theclinical situation

Availability Less than CT Readily available, Generally available,

usually 24 hrs/day but may be limited

at certain periodsdue to staffing

Table 2.2 Contraindications for MR imaging*

Absolute contraindications Relative contraindications

Cerebral aneurysm clip of unknown composition Claustrophobia

Possible metallic foreign body in the globe Critically ill patients

* Note: This table is not meant to be a complete list of all contraindications andany question should be referred to an MR imaging specialist

new contrast agents, such as ferumoxides (Feridex), which have been shown to prove lesion detection in certain cases

im-The selection of an ideal imaging technique for abnormalities of the biliary tree

is somewhat more complex In the evaluation of cholelithiasis or choledocholithiais,

US is clearly first choice due to its low cost, availability and accuracy If an tion is performed for suspected malignant disease, MRI is superior to both CT and

evalua-23Imaging of the Liver, Bile Ducts and Pancreas

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US since bowel gas will not interfere with evaluation of the distal common bile duct

or the identification of periportal and retroperitoneal adenopathy (Fig 2.5) atic imaging is best obtained with CT if contrast administration is not contraindi-cated If iodinated contrast cannot be administered, or the CT is equivocal, MRI isthe next test of choice MRI and CT are equivalent with regard to vascular encase-ment and adenopathy in the setting of pancreatic adenocarcinoma Microcysticadenomas may be better delineated with MRI, as the small cysts are more easilydepicted (Fig 2.6) Pancreatitis is best evaluated with CT especially if the patient isunstable or in critical condition

Pancre-The next section will review the more commonly encountered lesions of theliver, biliary tree and pancreas and their associated imaging features

The enhancement patterns of particular lesions are often critical to their terization Specific lesion types enhance during the arterial dominant phase(hypervascular) while others do not (Fig 2.3A) The most common lesions demon-strating arterial dominant phase enhancement include hepatocellular carcinoma,hepatic adenomas, focal nodular hyperplasia, and metastatic disease Other condi-tions include coexistent hepatic disease For example, patients with underlyingcirrhosis and a hypervascular lesion are considered to have hepatocellular carcinoma(HCC) until proven otherwise (Fig 2.3B) In addition to arterial dominant

charac-Table 2.3 Test selection based on body part

CT MR Imaging Ultrasound

Ranking is in order of preference (1 being the most preferred) When two

modalities are ranked first, it depends on the clinical indication for the study as to,which is chosen as discussed in the text

Fig 2.1A Hepatic cyst A) Ultrasound image through the left lobe of the liver onstrates a lesion (arrow) which is hypoechoic and thin walled with increasedthrough transmission

dem-Fig 2.1B A CT image of the same patient demonstrates two lesions Althoughthese lesions are low attenuation, Hounsfield units were greater than 10 and thusnondiagnostic for a cyst (Note a noncontrast CT was performed prior to the con-trast-enhanced scan to evaluate for enhancement.)

25Imaging of the Liver, Bile Ducts and Pancreas

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Fig 2.2A Hepatic hemangioma: Ultrasound examination demonstrates ahyperechoic mass (arrows) in the liver Although hemangiomas are hyperechoic, adefinitive diagnosis is often difficult

Fig 2.2B A CT examination of the same patient demonstrates a lesion (arrow) withperipheral nodular enhancement (curved arrow), characteristic for a hemangioma

Fig 2.2C An MRI examination in a different patient demonstrates a mass (m) which

is hyperintense on T2 weighted images The fluid is bright (please note CSF arrow).Gadolinium enhanced MRI (not shown) will also demonstrate the classic periph-eral nodular enhancement pattern when present

Fig 2.3A Metastatic disease ultrasound examination of a patient with a prior hepaticresection demonstrating a mass (arrow) in the residual left lobe The mass demon-strates a partial hypoechoic rim (arrowhead), a slightly hyperechoic portion, and acentral region which is hyperechoic (target)

27Imaging of the Liver, Bile Ducts and Pancreas

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Fig 2.3B A T2-weighted image demonstrates a mass (m) with a peripheral portion that

is slightly hyperintense to liver, and a central portion that is markedly hyperintense

Fig 2.3C Contrast enhanced T1-weighted MRI demonstrates heterogeneousenhancement (arrow) consistent with metastatic disease

enhancement, HCC typically demonstrate a pseudocapsule of compressed chyma Adenomas may demonstrate heterogeneous appearances from prior hemor-rhage, while focal nodular hyperplasia (FNH) may have a characteristic central scar.The scar in FNH enhances late after contrast, and on MRI is hyperintense onT2-weighted images

paren-Diseases metastatic to the liver display a variable appearance depending on theprimary underlying malignancy Sonographic evaluation may demonstrate multiplehypoechoic lesions, or lesions with a hypoechoic rim or halo pattern (Fig 2.4A).Metastases on CT scanning usually demonstrate a thick, irregular rim with enhance-ment (Fig 2.4A) MRI demonstrates lesions that are mildly hyperintense to liver(similar in signal to normal spleen) and these lesions tend to lose signal on moreheavily T2-weighted images (blend in with hepatic parenchyma) (Fig 2.4B) En-hancement patterns are somewhat variable but similar to CT findings; metastaticlesions tend to have irregular or rim enhancement

Biliary calculi are usually hyperechoic on US and generally demonstrate posterioracoustic shadowing (Fig 2.4C) These stones are most commonly seen on US in thebiliary tree and gallbladder On CT scanning, stones appear hyperdense and are easy

to delineate MRI of the biliary tree can be accomplished using magnetic resonancecholangiopancreatography (MRCP) which is a new, exciting technique combining rapidT2-weighted sequences with ultrafast images in multiple planes Calculi are hypointensecompared to surrounding bile and appear as filling defects in the duct

Precise imaging of malignant disease of the biliary tree is often more problematic.Small lesions may be difficult to identify especially in the presence of a biliary stent(Figs 2.5A, 2.5B) Patients with underlying disease of the biliary tract, or priorsurgical interventions, can present with scarring and stricture formation which can

be difficult to differentiate from a primary malignancy The presence of a stent orpneumobilia can add to the imaging difficulty

Pancreatic imaging is most accurately performed with CT, or MRI if the CT isequivocal US can obtain diagnostic images of the pancreas However, body habitusand bowel gas may obscure imaging of part or the entire pancreas (Fig 2.6) Adeno-carcinoma of the pancreas generally appears as a hypovascular mass best seen in thelate arterial phase of injection on both CT and MRI Vascular invasion or encase-ment is easily depicted as well as associated lymphadenopathy Both CT and MRImay be acquired to generate angiographic images of the surrounding vessels Neu-roendocrine tumors of the pancreas are usually hypervascular in nature on contrastenhanced images and are typically hyperintense on T2 weighted MR images Cysticneoplasms of the pancreas may also demonstrate characteristic appearances (Figs.2.7A, 2.7B) Calcifications, while seen on MRI are better appreciated on CT im-ages The size and nature of the cystic components are better delineated on MRI

29Imaging of the Liver, Bile Ducts and Pancreas

31Imaging of the Liver, Bile Ducts and Pancreas

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Fig 2.5B Axial image from a T2-weighted MRI demonstrates eccentric soft tissue(curved arrow) surrounding a narrowed common bile duct Note the plastic stent isnot appreciated as it is on the ultrasound

Fig 2.6 Pancreatic mass: Ultrasound examination demonstrates a normal appearingpancreatic head with a hypoechoic mass in the body/tail region (curved arrows)